1. Technical Field of the Invention
The present invention relates generally to electric motors. More particularly, the invention relates to an improved synchronous motor which includes a rotor having a field coil, permanent magnets, and a salient-pole structure.
2. Description of the Related Art
There is known, for example from Japanese Patent First Publications No. H11-89193 and No. 2006-121821, a synchronous reluctance motor which includes a rotor with a salient-pole structure and a stator with a stator coil.
In the salient-pole structure, a plurality of salient-pole portions are alternately arranged with a plurality of magnetic reluctance portions in the circumferential direction of the motor at an electrical angular pitch of (π/2). On the other hand, the stator coil creates a rotating magnetic field upon being energized with sinusoidal AC current. The rotating magnetic field is synchronized with rotation of the rotor, producing reluctance torque.
Moreover, defining a rotating coordinate system with d and q axes on the rotor, the reluctance torque can be represented as (Ld−Lq)×Id×Iq, where Ld is the d-axis inductance of the motor, Lq is the q-axis inductance of the motor, Id is the d-axis current of the motor (i.e., the d-axis component of the AC current supplied to the stator coil), and Iq is the q-axis current of the motor (i.e., the q-axis component of the AC current supplied to the stator coil). In addition, the rotating coordinate system, which rotates along with the rotor, is so defined that the inductance of the motor is highest in the d-axial direction and lowest in the q-axial direction.
Further, there are also known various types of salient-pole structures, such as a flux barrier type and a segment type.
However, with those known types, the efficiency of the motor is decreased due to the loss of AC current for creating the rotating magnetic field, thereby lowering the power density of the motor. Accordingly, it is desired to increase the salient-pole ratio (Ld/Lq) of the motor, thereby increasing the reluctance torque of the motor.
Moreover, in the known synchronous reluctance motor, all the magnetic flux transferred between the stator and the rotor is created by supplying the AC current to the stator coil. In general, the stator coil is configured to have a relatively small number of turns so as to decrease the reactance of the stator coil. Accordingly, to create all the magnetic flux, it is necessary to supply relatively heavy AC current to the stator coil. However, this will increase the copper loss (or ohmic loss) of the stator coil, thereby lowering the efficiency of the motor.
Alternatively, the magnetic flux can be created on the rotor side by employing a Lundell-type rotor core with a field coil wound thereon. However, in this case, the field coil is to be energized with DC current; further, the field coil would have a relatively large number of turns to decrease the DC current supplied thereto. Consequently, it would be difficult to quickly change the torque generated by the motor.
To solve the above problem, one may consider employing an IPM (Interior Permanent Magnet) synchronous motor which has both a salient-pole structure and permanent magnets embedded in the rotor; the IPM motor thus can generate both reluctance torque and magnet torque (i.e., torque generated by utilizing the magnetic flux created by the permanent magnets). However, in this case, to generate high magnet torque at a low speed of the motor, the permanent magnets are required to be capable of creating strong magnetic flux; this would make the permanent magnets expensive. On the other hand, at a high speed of the motor, the strong magnetic flux created by the permanent magnets would induce a large back electromotive force in the stator coil of the motor. To overcome the large back electromotive force, it is necessary to supply a high AC current to the stator coil; this would significantly increase the copper loss of the stator coil.